Training sequences are widely used in wired and wireless communication systems in the form of well designed sequences or waveforms known to both the transmitter and the receiver. The training sequence (TS) is used mainly for the purpose of channel estimation, and may also carry other information (e.g., signalling or user information, etc.) that can be detected, typically blindly, on the receiver side. The ultimate goal for the design of the TS is to optimize the channel estimation and signal detection performance in the receiver for a given set of constraints (e.g., channel delay spread condition, types of information carried over, expected operating signal-to-noise ratio and transmitter/receiver complexity, etc.). The IEEE 802.11ad specification, which provides a standard for the emerging WLAN/WPAN at the 60 GHz frequency band, defines a training signal called channel estimation field (CEF). FIG. 1 shows the frame structure of a Physical layer Protocol Data Unit (PPDU) 100 defined in the IEEE 802.11ad specification. The frame structure includes a short training field (STF) 102, a channel estimation field (CEF) 104, a header 106, a data packet 108 and a beamforming receiver/transmitter training (TRN-R/T) field 110. An automatic gain control (AGC) field may also be included. The channel estimation field (CEF) 104 is used for channel estimation and detection of modulation types used in header and data packets when they are transmitted over a physical layer of a network. For example, the modulation type may be orthogonal frequency division multiplexing (OFDM) or single carrier (SC). The modulation type is associated with the physical layer (PHY) type.
FIG. 2 illustrates the CEF sequences defined in the IEEE 802.11ad specification for wireless local area networks. FIG. 2A shows the sequence for single carrier (SC) modulation and FIG. 2B shows the sequence for orthogonal frequency division multiplexing (OFDM) modulation. The CEF sequence for SC modulation consists of a prefix sequence 200 which is a part of the STF 102, followed by eight concatenated, alternating, 128-bit Golay complementary sequences, 202, 204, 206, 208, 210, 212, 214 and 216, and a postfix sequence 218. The sequences Ga128 and Gb128 form a Golay complementary pair. Each sequence is modulated by a sequence of signs (+ or −). The prefix and postfix sequences (shown as shaded blocks in the figures) are cyclic extensions of the eight concatenated sequences. Similarly, the CEF sequence for OFDM modulation, shown in FIG. 2B consists of a prefix sequence 200′ which is also a part of the STF 102, followed by eight concatenated, alternating, 128-bit Golay complementary sequences, 202′, 204′, 206′, 208′, 210′, 212′, 214′ and 216′, and a postfix sequence 218′.
FIG. 3 shows graphs of periodic auto-correlation and cross-correlation functions of the CEF sequences specified in IEEE 802.11ad. FIG. 3A shows the periodic auto-correlation function R11 of the first (SC) sequence. Due to the CEF format shown in FIG. 2, the range of time lags for evaluation of auto-correlation and cross-correlation is limited to −128 to 128. The auto-correlation is zero for time lags in the range −128 to 128, indicated by the zone between the dashed vertical bars in the figure, except for time lag zero. FIG. 3B shows the periodic cross-correlation function C12 between the first (SC) sequence and the second sequence (OFDM). The cross-correlation is non-zero except for the single zero lag. Similarly, FIG. 3C shows the periodic cross-correlation function C21 between the second sequence and the first sequence, and FIG. 3D shows the periodic auto-correlation function R22 of the second sequence.
The CEF format in IEEE 802.11ad is designed for efficient channel estimation, but the format is not efficient for detecting the PHY type of a network's physical layer, since the cross-correlations, shown in FIG. 3B and FIG. 3C, have no zero-zone greater than a single lag.
It would be useful to provide improved training sequences that are efficient for both channel estimation and PHY type detection.